Tannins and other chromophoric extractives are naturally occurring materials occurring in wood-based products. These chromophoric materials are present in Western Red Cedar, Redwood Oak, and, White and Yellow Pine, to name a few. They are also present in wood composition products such as paper, cardboard, plywood, particle board, chip board, strand board and Masonite.
When protective film-forming finishes including paints, lacquers, and varnishes are applied to wood having a high content of water-soluble tannin, such as redwood, cedar, oak, or mahogany, the tannin often bleeds or migrates into the surface film, causing a dark stain in the film. Tannin staining, an undesirable process which results in aesthetic degradation and loss of decorative value of protective coatings, is a problem frequently encountered with coatings applied on wood substrates. It is observed usually as yellow-brown coloration or as randomly distributed brown-colored spots on coatings, and more particularly, on coated wood substrates exposed to high, typically condensing, humid conditions.
Water soluble tannins or tannic acids, natural compounds of complex and non-uniform composition, are the staining species involved, which are abundantly present especially in redwood substrates. Tannin staining includes several concurrent processes: water or vapor penetration of wood substrates, solubilization, diffusion into the coating and darkening of the deposited air-exposed tannin species, among others. It is significant to observe in this sense that the rate of staining is diffusionally controlled and its extent is significantly dependent on the substrate's tannin concentration.
By definition, staining inhibition implies a coating system's capacity to interact with dissolved tannin species and interfere with related diffusion processes. There are specialized pigment grade products known in the prior art as "blockers of tannin" or "stain inhibitors", which as functional components of water or solvent based paint formulations, provide such protective capacity to coatings systems applied on wood substrates. Sulfonium compounds have been disclosed as useful in blocking tannin migration in wood substrates (U.S. Pat. Nos. 3,636,052; 3,660,431 and 3,900,619). Highly cross-linked ion exchange resins have been incorporated into aqueous coating compositions applied to cedar and redwood type substrates (U.S. Pat. No. 3,494,878). Aqueous solutions of nonlinear polyalkyleneimines or linear, partially deacylated poly(N-acryl)alkyleneimines have also been used to inhibit tannin migration in wood substrates (U.S. Pat. Nos. 4,075,394 and 4,104,228 respectively). Amphoteric compounds of alumina, titania, zirconium, silica or zinc have been incorporated into certain composite pigments to inhibit the migration of water-soluble tannins through paint film (U.S. Pat. Nos. 3,852,087, 3,969,293, 4,021,398, 5,176,894, 5,529,811). In European Patent Application No. EP 622427 is disclosed a styrene-butadiene latex that exhibits tannin blocking properties. In U.S. Pat. No. 4,218,516, magnesium hydroxide is applied to a wood substrate, forming a leach-resistant magnesium hydroxide-tannin complex. In U.S. Pat. No. 5,460,644 is disclosed a zinc/ammonia salt that provides tannin stain-blocking properties. Also, in U.S. Pat. No. 5,527,619 is disclosed an organosilane-modified coating to block tannin staining in water-based and solvent-based coatings. Reactive pigments have also been incorporated into coatings, but reactive pigments have limitations such as causing instability problems, such as viscosity increase and polymer gelation, as well as being known to be environmentally unfriendly. Also, stain inhibitors containing specified metal hydrates as functionally active additives function essentially by reducing the permeability of coating systems and, thus display relatively limited tannin stain inhibitive capacity. U.S. Pat. No. 5,141,784 discloses a process for treating wooden substrates having leachable chromophoric substances by applying an effective amount of a carboxylic acid salt and/or a water soluble organic compound bearing one or more salt-forming amine groups and having a molecular weight of about 50 to about 300,000. U.S. Pat. No. 5,051,283 discloses a surface treating composition for wooden surfaces that inhibits leachable compositions from staining after-applied water-base coating comprising about 1% to about 25% by weight of an alkali metal salt of a monobasic or dibasic carboxylic acid and about 1% to about 50% by weight of a water soluble organic compound bearing one or more salt-forming amine groups and having a molecular weight of about 50 to about 300,000 and from about 25% to about 98% water. International Application Number PCT/US/00643 discloses a polymeric composition containing at least one acid-functional vinyl polymer and at least one amino-functional siloxane polymer. The blended polymeric compositions are able to provide various substrates with surface coatings that are characterized as quasi-crosslinked, alkali-soluble polymeric networks exhibiting enhanced surface properties. Also disclosed are methods of producing the blended polymeric composition. Japanese Patent Application J 03-064305A discloses a composition consisting of a carboxyl-group-containing, water-soluble resin, macromolecular aqueous dispersion and water-soluble silane coupling agent containing an amino group in the molecule in which the carboxyl-group containing, water soluble resin has an acid value of 30-800 and the amounts per 100 solid weight parts of the macromolecular aqueous dispersion of 0.2-2.0 weight parts of the aqueous resin and 0.-13 weight parts of the amino-group containing silane coupling agent are compounded.
It has been found, in accordance with this invention, that a beneficial tannin inhibiting effect is obtainable by utilizing a aqueous coating composition comprising a titanate coupling agent, preferably, an organotitanate. The process, in accordance with the this invention, for inhibiting tannin staining of coatings and paints involves applying to tannin-containing substrates, such as, for example, wood, cardboard, paper, wood composition particle board, Masonite, and the like. Although several systems for inhibiting tannin migration in tannin-containing wood substrates are known, none use or suggest the use of a titanate coupling agent as an effective tannin stain inhibiting agent. Also disclosed is a pigment composition comprising a titanate coupling agent.